This invention relates generally to bandpass filters and is particularly directed to a switched bandpass filter for processing a wide band RF signal particularly adapted for use in a frequency converter.
An electronic tuner such as used in a typical RF receiver includes up to three tunable circuits, i.e., a tunable circuit between the antenna input and RF amplifier, a tunable circuit between the RF amplifier and mixer, and a tunable circuit associated with the oscillator. Each of these tunable circuits is simultaneously tuned to a frequency representing the desired station, or channel, in forming ganged tuned circuits. When more than one octave of frequencies is to be covered by the tuner, in addition to tuning, band switching techniques are frequently used to selectively change some elements of each tunable circuit to accommodate the entire desired frequency band. Also many tuners or frequency converters, such as for example those used with CATV systems, operate without a selective front end or mixer protection. As the desired RF bandwidth in this case increases over an octave of frequencies, the number of intermodulation products and the possibility of crossmodulation products increases very fast imposing greater demands upon the tuner.
Prior art approaches to multi-band tuning and mixer protection have generally involved the use of varactor diodes and/or switching diodes in the tuning stages. The varactor diode approach makes use of circuit elements exhibiting a capacitance which is a function of the applied voltage. By varying the DC tuning voltage applied to the varactor diode, its capacitance changes and the frequency to which the resonant circuit is tuned may be adjusted within approximately one octave frequency range as desired. The limitation of the varactor tuning range to approximately 1 octave is overcome frequently by means of switching diodes forming together with the varactor diodes and other mostly inductive components a tuned and bandswitched network. Another limitation in this approach arises from an inherent characteristic of varactor diodes in that they exhibit nonlinear variations in capacitance with changes in the applied RF voltage. This adds more crossmodulation and intermodulation products to that caused by the tuner itself.
Another problem inherent in varactor tuned frequency converters with a relatively high IF frequency (upconverter) is the difficulty of frequency tracking between the RF and oscillator tuned circuits. To overcome this problem complicated analog or digital schemes are used. On the other hand, a simple switching diode approach to band selection generally involves the use of separate bandpass filters which are selectively switched in and out of circuit depending upon the frequency band desired. In this approach, one bandpass filter section is generally active, while the remaining filter sections are switched out of circuit and thus rendered inactive. The majority of circuit components are thus not in use at any given time and there is a high component count for the various filter sections. In addition, more than one switching diode is generally required for each frequency band to allow for the switching in and out of circuit of both the input and the output of each filter section so as to select the circuit for receiving a desired frequency band.
In order to reduce the cross- and intermodulation products in wide band CATV tuners and signal converters, double balanced mixers are frequently employed. However, with the CATV band being expanded into the UHF band, the practical limits of double balanced mixer performance within the CATV tuner have essentially been reached in terms of tradeoff between intermodulation and noise performance. With this large RF bandwidth, mixer intermodulation and noise performance determine the dynamic range of the CATV tuner. The noise figure for a typical frequency converter chain with a mixer, which typically exhibits a conversion loss from 6 to 8 dB, is generally in the range of 10 to 13 dB. Placing a feedback type preamplifier with 10 dB gain in front of the mixer would improve the system's noise figure to 5 to 7 dB with an associated 10 dB decrease of the signal converter's RF processing capability. The latter is generally unacceptable except if an electronic band switch is also placed in front of the mixer state. However, to date this capability is not yet available without employing overly complicated switching arrangements with large numbers of electronic components.
The present invention is therefore intended to overcome the aforementioned limitations of the prior art by providing a switched bandpass filter which is capable of operating over a large bandwidth, employs a minimum number of components by reusing most of the components in the inactive, or bandstop, sections of the filter, and utilizes only a single switching diode in each filter section. The switched bandpass filter of the present invention is particularly adapted for use in a low noise CATV or RF converter for reducing intermodulation and crossmodulation distortion in the mixer stage which is preceded by a wideband preamplifier.